I love the Flickr, but alas, my district does not, and I understand why. Part of the The Children's Internet Protection Act (CIPA) says that a school that wishes to be eligible of eRate funding must enforce a policy of Internet safety that includes the use of filtering or blocking technology. This Internet Safety Policy must protect against access, through computers with Internet access, to visual depictions that are obscene, child pornography, or (in the case of use by minors) harmful to minors.
So, Flickr, from time to time, may have some pictures that fit into the above categories.
Nonetheless, I have found a great use of Flickr (as I access it from home). I use the part of Flickr where users have chosen to offer their work under a Creative Commons license.
These images in the CC area, allow a person to use the images without fear of copyright issues, as long as one follows the licensing set forth by the owner. This is quite a bit different from using Google Images, where one never knows if they are violating copyright laws by using a picture in a presentation or elsewhere.